Very large volumes of gaseous hydrogen are commonly used in the chemical and petrochemical industries. Typically, these very high demands are met by providing gaseous hydrogen from a nearby hydrogen pipeline. There are serious financial risks for the supplier, and operational risks for the user, if the supply of gaseous hydrogen is interrupted. In such precarious conditions, it is beneficial to have a storage facility connected to the pipeline to buffer such potential disruptions. If such an interruption is due to an unscheduled hydrogen production plant outage, this disturbance may take many hours or days to remedy. Hence the storage facility will need to be very large to be useful. One alternative, if it is geologically convenient, is an underground salt cavern.
However, it is reported in the literature that high purity (e.g., 99.99%) hydrogen storage within salt caverns presents several challenges, (See U.S. Pat. Nos. 8,690,476 and 9,284,120, the entire contents of which are both hereby incorporated by reference.) This literature states that, for example, storing large quantities (e.g., greater than 100 million standard cubic feet) of pure (e.g., 99.99%) gaseous hydrogen in underground salt caverns consisting of a minimum salt purity of 75% halite (NaCl) or greater without measurable losses is difficult based on the properties of hydrogen. It is noted in this literature that hydrogen is the smallest and lightest element within the periodic table of elements, having an atomic radius measuring 25 pm+/−5 pm.
Further, hydrogen is flammable, and therefore a very dangerous chemical if not handled properly. The literature states that salt caverns consist of salt that have various ranges of permeability (e.g., 0-23×10^−6 Darcy) that if not controlled properly could easily allow gaseous hydrogen to permeate through the salt and escape to the surface of the formation. If the stored hydrogen within an underground salt formation was to escape and permeate through the salt formation to the surface, a dangerous situation could arise with fatality potential or immense structural damage potential. The literature also observes that high purity hydrogen is typically considered one of the most difficult elements to contain within underground salt formations. (See U.S. Pat. No. 8,690,476)
While the in-situ salt formation within a salt cavern may be essentially gas impermeable, the process of solution mining the salt and forming the cavern, even though continuously filled with pressurized fluid, is known to introduce fractures of various sizes in the crystal structure of the salt. While the salt naturally has low permeability and porosity rendering it largely impermeable to hydrocarbons, it is reported in the literature that the salt is significantly more prone to very high purity hydrogen permeation by virtue of hydrogen's small atomic radius. (See U.S. Pat. No. 8,690,476, column 1, lines 56-64)
This same prior art has reported that pressures in excess of 1.0 psi per linear foot within salt caverns (Hcavern) is the technological pressure limit for the state of the art for substantially confining hydrogen when storing very high purity hydrogen rather than other products with larger molecular sizes such as natural gas. See, for example, U.S. Pat. No. 8,690,476, which references instant FIG. 9 (Prior Art FIG. 4C), wherein it states:
“FIG. 4C, on the other hand, is indicative of one or more cracks or fractures along the salt walls 203 which can potentially form when the stored hydrogen 4 is maintained in the cavern 3 at a pressure substantially greater than about 1 psi per foot of cavern depth. The cracks are sufficiently large to allow hydrogen to leak therethrough. By way of comparison, the hydrogen leakage across the salt walls 203 occurs at a higher flow rate than the hydrogen seepage in FIG. 4A by virtue of the cracks creating larger flow paths. The scenario of FIG. 4C is representative of the stored hydrogen 4 being stored above the upper limit.”
U.S. Pat. No. 8,690,476 defines the term “cavern depth” with reference to instant FIG. 10 (Prior Art FIG. 2), as follows:                “The cavern depth that starts at the top of the salt and ends at the bottom of the salt cavity is denoted as “d” and is defined as the vertical distance spanning from the top-most portion 204 to the bottom-most portion 207 of the salt cavern 3.”        
For a given salt cavern volume, an increase in the maximum allowable gas storage pressure will result in the ability to store a greater number of gas molecules. If one doubles the storage pressure, the number of standard cubic feet of gas that can be stored in the same volume is essentially doubled. If one triples the storage pressure, the number of standard cubic feet that can be stored in the same volume is essentially tripled. The factor that keeps this from being a direct ratio is the compressibility factor, which for hydrogen increases by about 7% when tripled, at the pressures at which the gas is typically stored. Salt cavern mining and construction is quite expensive. Therefore, there is significant financial and commercial advantage for finding a safe and economical method for increasing the storage capacity of very high purity hydrogen gas per unit of physical volume in an existing underground salt cavern or when constructing a new salt cavern storage facility.
The inventors have found that, under appropriate conditions, the current technological limit of “1 psi per foot of cavern depth” as defined above in the literature may be significantly exceeded resulting in greatly improved economics for hydrogen salt cavern storage.